A promising strategy for the development of anti-cancer drugs is to suppress activation of oncogenic proteins such as Ras superfamily members. Functional Ras proteins require the addition of an isoprenoid lipid via a process directed by a C-terminal sequence termed the CaaX motif (1, 2), in which a cysteine (C) residue is followed by a dipeptide (aa) that is usually aliphatic and a variable residue (X) that dictates the prenyl group added. An X residue of Ser, Met, Gln, Cys, or Ala directs addition of the 15-carbon farnesyl lipid, while a Leu residue can direct modification by the 20-carbon geranylgeranyl isoprenoid (3). Following addition of the isoprenoid, most CaaX proteins are further processed by the Rcel protease, which removes the -aaX residues, and isoprenylcysteine carboxymethyltranferase (Icmt) to yield a protein containing a C-terminal isoprenylcysteine carboxymethylester (4).
The CaaX prenyltranferases protein farnesyltransferase (FTase) and protein geranylgeranyltransferase type I (GGTase-I)(5) add either a 15 carbon farnesyl group or a 20 carbon geranylgeranyl group, respectively, to the cysteine found within the CaaX motif. The prenyltransferase holoenzyme consists of an alpha subunit that is shared between the two forms of enzymes and a distinct beta subunit that binds substrates and thereby also confers prenyl group specificity (1). FTase substrates include Ras GTPases, nuclear lamins, several protein kinases and phosphatases, as well as other regulatory proteins (5). GGTase-I substrates include Rac and Rho GTPases and the gamma subunits of most heterotrimeric G-proteins (6). A structural bioinformatics analysis of the human genome has indicated that roughly 130 proteins are modified by the two enzymes, with substrates being divided nearly equally between FTase and GGTase (3).
The realization that many CaaX proteins, most notably Ras family members, are involved in pathologies such as cancer, inflammation and viral infectivity (4, 7, 8, 9), has spurred efforts to develop inhibitors of CaaX processing as a rational approach to therapeutic development. The major effort in this regard has been development of FTase inhibitors termed FTIs. FTIs developed rapidly from early CaaX peptide mimics (10) into the small organic ligands that have now shown efficacy in clinical trials, particularly hematologic and breast cancers (11, 12). FTI development was unusually rapid as multiple groups developed structurally distinct drugs that all demonstrated similar biological/pharmacological properties in preclinical models and unambiguously indicated that drug action was due principally to inhibition of FTase. These results however, have been overshadowed by unexpected clinical shortcomings, particularly against solid tumors (13, 14). One postulated explanation for this clinical limitation is the increased geranylgeranylation of protein such as K-Ras when FTase activity is inhibited, a processes referred to as alternate prenylation (15, 16, 17).
The discovery of alternate prenylation, coupled with the increasing evidence for involvement of geranylgeranylated proteins in pathological processes such as cancer, inflammation, and viral infectivity has led to increased interest in therapeutic targeting of GGTase-I. Although development of FTIs has garnered considerably more interest than that of GGTIs, several peptidomimetic GGTIs have been described. These include aminobenzoic acid derivatives such as GGTI-298 and GGTI-2154 (18, 19) and benzoyleneurea-based compounds (20). Studies with these compounds have revealed a number of consequences of cellular exposure to a GGTI. Administration of GGTI to cells can cause cell cycle arrest at G0/G1, and this effect appears to be mediated by inactivation of CDK2/4 through the p21/p15 kinase inhibitors downstream of Rho (21, 22). GGTIs are also potent stimulators of apoptosis in both normal (23, 24) and transformed cell lines (25, 26).
The complex nature of CaaX protein localization and function leaves multiple points where disregulation can occur, and therefore multiple points for therapeutic intervention are possible. Although the efficacy of prenyltransferase inhibitors in treating cancer is now considered quite genuine, if still against a limited number of cancers, the rational application of protein prenyltransferase inhibitors is growing more complex as the roles for multiple GTPases and other CaaX containing proteins are elucidated.
Glaucoma is a leading cause of blindness. It is a progressive optic neuropathy often caused by elevated intraocoular pressure (IOP) consequent to abnormally high resistance to aqueous humor drainage via trabecular meshwork (T.M.) and Schlemm's canal (SC). The conventional route of aqueous humor outflow through T.M. and SC is generally thought to be the major pathway for the drainage of aqueous humor from the eye. Although there exist various medications to lower intraocular pressure in glaucoma patients, there is no specific drug which selectively targets the TM pathway.
Age-related macular degeneration (AMD) causes progressive impairment of central vision and is the leading cause of irreversible vision loss in older Americans. The most severe form of AMD involves neovascular/exudative (wet) and/or atrophic (dry) changes to the macula. Although the etiology of AMD remains largely unknown, implicated risk factors include age, ethnicity, smoking, hypertension, obesity and diet. Extracellular protein/lipid deposits (drusen) between the basal lamina of the retinal pigment epithelium (RPE) and the inner layer of Bruchs' membrane are associated with an increased risk of progressing to an advanced form of AMD, either geographic atrophy or exudative disease. The presence of large and indistinct (soft) drusen coupled with RPE abnormalities is considered an early form of the disorder and is often referred to as age-related maculopathy (ARM).
There is a continuing need in the art to develop drug treatments for alleviating the symptoms of glaucoma and age-related macular degeneration.